1. Field of the Invention
The present invention generally relates to self-oscillating power supplies with adjustable power control and, more particularly, to electronic, self-oscillating discharge lamp ballast with lamp dimming control.
2. Description of the Prior Art
The increased efficiency of gas-discharge lamps, such as well-known fluorescent tubes, over incandescent light sources has long been appreciated. Such gas-discharge light sources generally produce far less heat and thus have a much higher efficiency of energy conversion to light. Overall efficiency of such lamps in a given application may also be increased by the fact that removal of heat is generally unnecessary. However, some problems have been presented in the past in regard to light spectral content and the ability to control the amount of light output from a given gas-discharge lamp which are not presented by incandescent light sources.
Nevertheless, the efficiency gains to be realized over incandescent sources has led to significant advances in both of these areas and, in many cases, gas-discharge and fluorescent lamps have been fabricated and marketed as direct replacements for incandescent lamps. However, the electrical characteristics of gas-discharge lamps are much more complicated than those of incandescent lamps (the latter being essentially a slightly variable resistor) and known arrangements for controlling light output of gas-discharge lamps have been correspondingly complex and expensive.
More specifically, only slight brightness control is available from traditional magnetic ballasts for gas-discharge lamps and electronic ballasts are generally preferred at the present time to provide a wider range of brightness control as well as the advantages of small size, light weight, high luminous efficiency, long life and the absence of light flicker or audible noise. The capability of dimming or brightness control is currently considered crucial in energy-saving lighting systems which studies have shown can provide in excess of fifty percent energy savings. In commercial environments dimming control systems can increase productivity by allowing employees to have personal control over workspace lighting, for example, to reduce glare at computer workstations. Dimming systems can also provide flexibility and achieve aesthetic effects for different activities and add versatility to architectural spaces, such as by visual partitioning, and visually enhancing objects and features of the spaces.
Unfortunately, as alluded to above, electronic ballast providing for dimming control is expensive and complex. Self-oscillating drive is usually employed to reduce control complexity and cost as much as possible. However, the operation of known self-oscillating circuits is, by their nature, load dependent and thus difficult to control and very sensitive to variations of the load, components of the circuit and line input voltage. This is because variations in the load, components of the circuit and/or line voltage alter the self-oscillation frequency and thus change lamp power, often by a larger factor than the variation from nominal parameter values. For example, the lamp power may vary xc2x115% for a xc2x110% variation in line voltage. Such a variation substantially exceeds the xc2x110% lamp power limit and may shorten lamp lifetime. These problems are not limited to control of gas-discharge lamps but are common to self-oscillating arrangements in power supplies and converters, generally, including those of buck, boost, buck-boost, fly-back types as well as forward converters, push-pull converters, symmetrical or asymmetrical half-bridge converters and full bridge converters.
Another complication is that of lamp current crest factor (CF) which is the ratio of of the peak of the lamp current to the rms value. For high frequency ballast, the CF is the peak of the modulated envelope to the rms value and plays a significant role in ballast design. Generally, the higher the CF, the shorter the lifetime of a fluorescent lamp. A pure sine wave without modulation has a crest factor of 1.4 and a triangular wave has a crest factor of 1.7. ANSI specifications recommend a maximum CF of 1.7. Since some electronic ballasts such as charge-pump power-factor-correction (CPPFC) electronic ballasts use modulated resonant components to achieve power factor correction, the lamp current CF is usually larger than 1.7. Therefore, frequency modulation is usually employed to improve CF even though it is subject to the difficulties discussed above.
It is therefore an object of the present invention to provide a simplified and relatively inexpensive electronic power supply/converter and/or ballast for gas-discharge lamps which provides easily controlled adjustability of power regulation with high efficiency and good lamp current crest factor.
It is another object of the invention to provide an effective technique and circuit for achieving frequency modulation for improving the crest factor of circuits using resonant components for power factor correction such as charge-pump power factor correction electronic ballasts and the like.
In order to accomplish these and other objects of the invention, a power supply such as a ballast for a gas discharge lamp is provided comprising an inverter circuit, a resonant matching circuit, and a converter circuit for determining switching frequency of the inverter circuit substantially independent of a load, whereby power coupled to a load through the resonant circuit is controlled.
In accordance with another aspect of the invention, a method of controlling high frequency electrical power delivered to a load from a self-oscillating power supply is provided including steps of controlling magnetizing current in a transformer of an inverter circuit to control switching frequency of the inverter circuit, and coupling an output of said inverter circuit to a load through a resonant circuit.